The present invention relates to a unit for preparing leaves of paper material.
The invention finds application to advantage in cigarette making machines, and in particular filter tip attachments by which single tipping papers are cut from a continuous strip decoiled from a roll; reference is made explicitly to this art field in the following specification, albeit implying no limitation in general scope.
Filter tipped cigarettes are assembled generally by interposing a double length filter plug between two axially aligned cigarette sticks, then joining the filter to the sticks on either side by affixing a previously gummed tipping paper, and ultimately cutting the double assembly in half to create two single filter cigarettes.
Employing prior art methods, tipping papers are obtained from a continuous strip of paper material caused to advance along a predetermined feed path, first through a gumming device by which adhesive is applied to one face of the strip, and thereafter to a cutting device by which the strip is divided up into single leaves of predetermined length.
The cutting device operates downstream of the gumming device, relative to the direction followed by the advancing strip, and comprises two rollers contrarotating tangentially to one another about parallel axes, one with an aspirating surface by which the strip is drawn forward; each roller is equipped with respective blades arranged angularly at uniform pitch around its peripheral surface.
Each blade presented by one of the two rollers will interact typically with a corresponding blade of the other roller in such a way as to sever the strip along transverse lines, executing a so-called scissor cut. Accordingly, each blade of one roller combines with a blade of the other roller to create a scissor device which, when the rollers are driven in rotation, is designed to separate successive tipping papers by shearing point-to-point across the strip.
After the cut, the tipping papers are held on the surface of one of the two rollers, and more exactly the suction roller, before being transferred in succession to a station where each is offered to an aforementioned assembly composed of a double length filter plug and two cigarette sticks.
For the operation of joining the assemblies to be performed correctly, the tipping papers cut from the strip must be suitably distanced one from the next; this is accomplished by causing the suction roller to rotate at a peripheral speed greater than the linear speed of the advancing strip.
Because the scissor cut is generated by degrees, signifying that the separation of the single paper does not occur instantaneously but only after a given interval of time, the difference between the peripheral speed of the suction roller and the linear speed of the advancing strip prevents the papers being detached cleanly from the strip. As a result of the traction force exerted by the suction roller, in effect, the advancing strip is subjected to a pulling action which the narrowing portion of material gradually becomes unable to withstand as the final stage of the scissor cut is reached and tension in the uncut portion of the strip becomes excessive, so that the tipping paper separates by tearing, before a clean cut can be completed. The papers detached in this manner cannot therefore be utilized as they are marred by irregularities along the line of the cut at the point where the final separation occurs.
To overcome such drawbacks, prior art methods include the expedient of inserting a cyclically activated diverter between the gumming device and the cutting device, by which the tension of the strip is varied at intervals matching the stroke frequency of the cutting device. More exactly, the diverter serves to relax the tension of the strip during the cutting stroke
With higher and higher strip feed speeds being adopted to accommodate increasingly fast production tempos, it can happen, due to the inherent elastic properties of the strip of paper material and the vibration generated by the diverter in the length of strip extending between the selfsame diverter and the cutting device, that the steps of relaxing the tension and making the cut will slip out of phase, and there is a risk of the material tearing along the final part of the cutting line.
The object of the invention is to provide a unit for preparing leaves of paper material such as will be unaffected by the aforementioned drawback.